1. Field of the Invention
The present invention relates to a technique for removing a foreign substance such as dust adhering on the surface of an optical member such as an image sensor or optical filter which is built in, e.g., a single-lens reflex digital camera and arranged on the focal plane or near the focal plane.
2. Description of the Related Art
As a conventional problem, if a foreign substance such as dust exists near the focal plane of the photographing lens of a single-lens reflex digital camera of a lens interchangeable type, the image sensor senses the shadow of the foreign substance.
Such dust is thought to enter from the outside upon lens interchange or be fine abrasion powder of, e.g., a resin or metal serving as the structural member of the shutter or mirror as it operates in the camera. The dust generated due to these factors sometimes enters especially between a cover glass for protecting the image sensor and an optical filter such as an infrared cut filter or optical low-pass filter arranged on the front surface of the cover glass. To remove the dust, it is necessary to disassemble the camera. Hence, it is very effective to form a sealed structure to prevent the dust from entering between the cover glass and optical filter of the image sensor.
However, another problem still remains unsolved. That is, if dust adheres on the surface of the optical filter on the opposite side (lens side) of the image sensor side close to the focal plane, the image sensor senses the shadow of the dust. To solve this problem, there is proposed a structure in which an optical filter capable of vibration is arranged on the front surface of the image capturing unit (Japanese Patent Laid-Open No. 2002-204379). A piezoelectric element vibrates this optical filter to remove the dust adhering on the optical filter. The camera structure as disclosed in Japanese Patent Laid-Open No. 2002-204379 can remove the dust adhering on the outermost surface of the dustproof structure (e.g., the surface of the optical filter) without detaching the lens and disassembling the camera.
Unfortunately, the above-described conventional dust removal mechanism poses the following problems.
That is, the dust particles adhering on the optical filter varies in size, composition, and shape, and position on the optical filter, where these dust particles are likely to adhere cannot be predicted. For this reason, the conventional dust removal mechanism sometimes fails to sufficiently remove the dust depending on, e.g., its size and adhesion position.
In addition, since the piezoelectric element itself for generating vibration is displaced only slightly, the conventional dust removal mechanism sometimes fails to efficiently remove the dust even by vibrating the optical filter using the displacement amount of the piezoelectric element alone. To increase the vibration amplitude of the optical filter, it is generally necessary to vibrate the optical filter at its natural resonance frequency. Applying a high voltage to the piezoelectric element also makes it possible to increase the displacement amount of the piezoelectric element so that vibration having a larger amplitude acts on the optical filter. However, an attempt to efficiently remove dust particles that take various forms leads to an increase in the apparatus size. At the same time, such attempt increases energy consumption by the power supply and prolongs the removal operation time.